Metal parts, for example, those used in jet engines are generally required to meet very precise tolerances. Damage to metal parts during use or during machining where a part is overmachined can prevent the part from falling within the set tolerances and require that the part be repaired or replaced. To repair such damage, new metal must be brazed to the surface of the part.
The new metal needs to have a composition similar to the base metal. Thus, diffusion braze fillers are combined with powder base metal to provide a composition which brazes to the base metal at a temperature lower than the melting point or softening point of the base metal.
Typically, this was done by forming a slurry which includes the powdered base metal, powdered diffusion braze filler and a binder which could be, for example, a methacrylate binder, an alginate binder or the like. These systems provide acceptable results. However, well defined geometries needed for some repairs were very difficult to obtain.
Further, slurries such as these are difficult to use. The binder system must be initially mixed. Then the precise amount of base metal and diffusion filler must be combined. This has a very limited shelf life. It cannot be mass produced for sale and subsequent use. It must be prepared by the actual user which creates the potential problem of human error.
Also, the boron typically used in the braze alloy could localize or puddle on the surface of the part. This weakens the base metal and can destroy the part. Slurries are also difficult to conform, resulting in poor ability for large build-up repairs.
Also, with oxygen sensitive alloys such as those that include titanium, aluminum, hafnium, and chromium, heating above 800xc2x0 F. can cause oxide formation. These oxides are not normally reducible in brazing furnaces. Most braze furnaces are designed to either operate in a vacuum or in a hydrogen atmosphere. However, there is frequently a trace amount of oxygen remaining in the furnace that can react with these metals. To avoid this problem, such alloys containing these oxygen sensitive metals are nickel coated prior to base metal repair. This nickel precoating is undesirable simply because it requires an extra step or even two extra steps frequently requiring masking of portions that are not to be nickel coated.
Accordingly, it is an object of the present invention to provide a base metal repair which is not a slurry and which does not require nickel precoating.
Further, it is an object of the present invention to provide such a base metal repair which provides precise dimensional repairs.
The present invention is a base metal repair tape which is a plural layered tape. At least one inner layer is formed from base metal powder bonded together by a binder such as fibrillated polytetrafluoroethylene. At least two layers are diffusion braze alloy also bonded together by a binder such as fibrillated polytetrafluoroethylene. The outer layers sandwich the inner layer. These layers are bonded together and can be placed directly on the repair area as a tape. During the thermal repair cycle, the diffusion braze alloy would melt and infiltrate the base metal tape from both sides providing a repair which would essentially keep its dimensional integrity as well as have a greater per volume density of basis metal.
Where geometries are complex, stresses placed on the tape during repair can cause the tape to lift away from the repair surface during the thermal processing. However using a three layered tape with a diffusion layer contacting the repair surface enhances wetability and reduces the possibility of tape movement. This lower layer is preferably maintained relatively thin to prevent damage to the repair surface. Since this lower layer is thin, there is little likelihood that boron will damage the base metal.
This tape provides a cleaning ability to repair surface as well as the base powder particles themselves. This can be extremely important where the base metal and the base metal powder of the repair contain high levels of aluminum and titanium which are potential oxide formers and potentially a threat to the success of the repair.
Further, the tape""s cleaning ability reduces or eliminates the need for nickel plating over the part in the repair area due to the enhanced brazability.
Further, the resulting brazed metal structure is enhanced by the separation of the base metal powder and the diffusion metal powder. This is because using the current method, the distance between the base metal powder particles is reduced due to the separation of the components in the multi-layer tape. This creates a repair of more nearly part-like mechanical properties because of the higher base metal content per unit volume.
By holding at brazing temperature or slightly below for an extended period of time (e.g., 2 hours), the softening or melting point of the repair can approach the softening or melting point of the base metal, resulting in a higher quality repair, more closely approaching base metal properties.
The reason for the improvement stems from the outward diffusion of melting point suppressant away from the repair area, significantly reducing its concentration in the repair, and slightly raising it in the surrounding base metal.
Further, the flexible and pliable nature of the tape allows repairs of complex geometries to be easily addressed and promotes the easy manufacture of preforms for repairing multiple identical parts. Finally, after the polytetrafluoroethylene resin has been evacuated, the base metal tape geometry structure remains intact providing for near drawing dimensioned preforms reducing timely and costly machining of the repair area after repair thermal cycle.
In an alternate embodiment of the present invention, the tape can be formed with multiple alternating layers of base metal tape and diffusion braze alloy forming a tape which is greater than one-half inch thick. This can be cut to size to form intricate parts which can be machined to size and used.
These advantages of the present invention as well as others will be appreciated further in light of the following detailed description and drawings in which: